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    SubjectRe: [PATCH v2 12/28] thunderbolt: Add functions for allocating and releasing hop IDs
    On Tue, Feb 12, 2019 at 01:43:33PM +0100, Lukas Wunner wrote:
    > On Mon, Feb 11, 2019 at 10:30:43AM +0200, Mika Westerberg wrote:
    > > On Sun, Feb 10, 2019 at 01:13:53PM +0100, Lukas Wunner wrote:
    > > > If there are two Macs at the ends of the daisy-chain with Thunderbolt
    > > > devices in-between, the other Mac may already have established tunnels
    > > > to some of the devices and therefore has occupied hop entries in the
    > > > devices' path config space. How do you ensure that you don't allocate
    > > > the same entries and overwrite the other Mac's hop entries, thereby
    > > > breaking its tunnels?
    > >
    > > If the other Mac has enumerated the device (set the upstream port,
    > > route, depth) then the other Mac cannot access the device. You get an
    > > error (we deal with that in the later patch in the series when we
    > > identify XDomain connections). The Hop ID allocation is only relevant in
    > > a single domain. Crossing one needs to have protocol such as we have in
    > > case of ThunderboltIP to negotiate Hop IDs used in the link between two
    > > domains.
    >
    > Understood now, thanks. (Well, in part at least.)
    >
    > It looks like there's a race condition currently in tb_switch_configure()
    > wherein two machines on the daisy chain may write the config simultaneously
    > and overwrite each other's changes. Isn't there some kind of synchonization
    > mechanism available to prevent such an outcome?

    AFAICT that's expected. The host that first enumerated the device wins.

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